Process for preparing intermetallic beryllides

ABSTRACT

INTERMETALLIC BERYLLIDES ARE PREPARED BY INDUCTION MELTING PARTICLES OF BERYLLIUM AND A REFRACTORY METAL, WHICH PARTICLES ARE LARGER THAN 50 MESH AT A TEMPERATURE OF AT LEAST ABTOU 3000*F. UNDER SLIGHT PRESSURE OF AN INERT ATMOSPHERE, PROCESSING THE RESULTING MELTED INGOT INTO A POWDER, TREATING THE POWDER WITH A AQUEOUS SOLUTION OF A DISODIUM SALT OF ETHYLENEDIAMINETETRAACETIC ACID HAVING A PH BETWEEN ABOUT 4.0 AND 6.0 FOLLOWED BY SINTERING THE POWDER TO FORM A SOLID BERYLLIDE PRODUCT.

United States Patent O 3,595,641 PROCESS FOR PREPARING INTERMLETALLIC BERYLLIDES Milton S. Roush, Claude R. Wheeler, and Muneo Fn n, Phoenix, Ariz., assignors to The Garrett Corporation No Drawing. Filed Oct. 10, 1968, Ser. No. 766,612 Int. Cl. C22c 25/00 U.S. Cl. 75.5 5 Claims ABSTRACT OF THE DISCLOSURE Intermetallic beryllides are prepared by induction melting particles of beryllium and a refractory metal, which particles are larger than 50 mesh at a temperature of at least about 3000 F. under slight pressure of an inert atmosphere, processing the resulting melted ingot into a powder, treating the powder with an aqueous solution of a disodium salt of ethylenediaminetetraacetic acid having a pH between about 4.0 and 6.0 followed by sintering the powder to form a solid beryllide product.

BACKGROUND OF THE INVENTION A great number of metallic alloys have been studied for optimum properties necessary for use in high temperature environments. Especially critical in aircraft turbine engine requirements are those metallic alloys having light weight and oxidation resistance and high stress rupture strength at the temperatures to which the metals will be subjected. Other desirable properties include high specific heat and thermal conductivity, ductility as well as acceptable coefiicients of thermal expansion and high modulus of elasticity. One group of metallic compositions found to have such characteristics comprises the intermetallic compounds of beryllium and the transition elements and particularly those having the general formula Where M is an element such as columbium, tantalum, titanium, vanadium, zirconium, hafnium and chromium. However, a major difficulty in preparing bodies and parts from the intermetallic beryllides is in fabrication of products having consistent and controllable qualities. In producing such bodies it is necessary to avoid oxidative contamination as well as to control such factors as material density and homogeneity. A previously disclosed method of producing beryllides consists of using finely divided particles of the two metals to be combined, i.e. 50 mesh or finer corresponding to a sieve opening of about 0.01 inch or less, in order to provide more intimate intermixture prior to heat treatment. Such a process is disclosed, for example, in US. Pat. No. 3,150,975. Yet, it is found that the use of finely divided particles or powders results in a number of disadvantages. Due to the small particle size and concomitant large surface area that is exposed, contamination by oxidation results yielding a product having inferior properties. In addition, small particles can only be induction heated with great difiiculty due to the high frequencies required to melt the materials as compared to larger particles. In previously known beryllide processes, the metallic constituents have been reacted at relatively low temperatures. However, it is also found that in low temperature processing, as the metals begin to combine, the beryllides of intermediate compositions are formed. Accordingly, beryllides such as M Be MBe etc. are formed initially at low temperatures which possess relatively high melting points. In this manner, portions of the reacting molecules become tied up and are essentially prevented from further reaction. Thus, although analysis of the final products indicates overall desired percentages of the individual metals, actual homogeneity of the desired compound or compounds throughout the body of an intermetallic is not realized. The present invention is directed to a process in which highly homogeneous intermetallic beryllides are formed as will be more fully described hereinafter.

DESCRIPTION OF THE INVENTION The initial phase of preparing the intermetallic beryllides according to the process of the invention comprises melting relatively large particles of the two metals desired. The metals used to combine with beryllium include columbium, titanium, tantalum, vanadium, zirconium, hafnium and chromium. It is especially critical that the size of the metallic particles used in the melting phase, during which the intermetallic compounds are prepared, be larger than 50 mesh. Useful particle sizes are preferably those having an average particle diameter greater than about 0.1 inch (8 mesh) in order to avoid undue oxidative contamination. The maximum particle size which can be used is not especially critical and is limited only by the size of the vessel in which melting takes place. Particle sizes having an average diameter of between about 0.5 and about 4 inches are especially preferred since they can be easily handled and will have a minimum of exposed surface area as compared to smaller particles. Further, such particles can be readily melted utilizing induction methods. It is also necessary that the temperatures to which the particles are subjected be above at least about 3000 F. and preferably above about 3300 F. in order to insure that complete and rapid melting occurs. At such temperatures as the beryllium melts, it reacts with the other metal present to form an intermetallic beryllide compound by exothermic reaction. Accordingly, as the melting temperatures of beryllium are exceeded, the heat of reaction is continuously generated until both metals are completely melted. It is also necessary to avoid significant vacuum conditions during the heating stage in order to minimize beryllium evaporation at the high temperatures required. It is preferable to utilize pressurized conditions, suitable pressures being between about 5 and about 10 p.s.i. absolute. The presence of oxygen, nitrogen or other contaminating gases which will combine with the metals present must also be avoided in order to substantially eliminate their reaction with the intermetallic produced. Inert atmospheres such as helium, hydrogen, argon and the like are suitable for the melting operation. Once the metals are completely melted, some stirring may also be beneficial in order to assure complete mixing of the metals and improve homogeneity.

Utilizing the above method of melting the beryllium and refractory metals yields essentially homogeneous intermetallic beryllides of the general composition desired. By merely melting together beryllium and refractory metals of proper stoichiometric proportions, substantially pure intermetallic beryllides of the composition between MBe and MBe are produced. The melting phase may be carried out in a container or vessel which will not be affected by the melting metals and which, in turn, will resist cracking or contamination of the intermetallic. Suitable materials include, for example, beryllium oxide, alumina, zirconia, and the like. It may also be desirable to utilize a larger back-up crucible in which the melt crucible is placed where cracking of the latter is of concern.

The melting chamber should be carefully purged with the inert atmosphere prior to the melting operation in order to insure that undesirable gaseous contaminants have been evacuated. Purging may consist of continuously charging the inert gas to the chamber for a period of time and may also include an initial evacuation of the chamber prior to the purging operation.

Once the metallic constituents have been completely melted and satisfactorily mixed, power to the induction coil is turned off and the molten charge is allowed to cool. Prior to cooling, where desirable, small amounts of other metals may be added to the intermetallic melt in order to improve its properties for specific use requirements. In the alternative, these materials which will be more fully disclosed hereinafter may be added to the powdered beryllide prepared from the cooled melt.

To produce bodies or parts from the intermetallic beryllides, it is necessary to further process the cooled ingot into a powder consisting of particles having a size preferably finer than about 200 mesh and more preferably between about 400 and 800 mesh. Suitable means comprise a crushing and/or grinding operation including, for example, the use of a ball mill whereby the particle sizes of the powder are significantly reduced to the desired extent. Particles which are too large to meet the desired size may be separated by screening and further processed. This grinding and milling operation results in iron contamination which must be removed. Although a portion of the iron may be separated mechanically by magnetic means, this is generally insufiicient and additional treatment is necessary. It is to this additional step that a further embodiment of the invention is directed. Accordingly, the finely divided intermetallic beryllide powder particles are treated with a sequestering or chelating agent in order to further remove iron impurities. This is accomplished by slurrying the powder in an aqueous solution containing the chelating agent. An especially preferred chelating agent consists of a partial salt of ethylenediaminetetraacetic acid, which in solution has a pH of between about 4.0 and about 6.0. Preferred partial salts are those of alkali and alkaline earth metals. It is important that the proper partial salt be utilized since, if the solution is too acidic, a substantial portion of the finely divided intermetallic may be dissolved. Further, where a trior tetra-salt is used, sufiicient sites on the chelating agent are not available for complexing the iron impurities. Especially preferred is the disodium salt of the acid. This phase of the process may be accomplished under ambient conditions and the powder is preferably stirred into the chelating solution for a number of hours. The powder is then separated by filtering, centrifuging or other suitable means. The filtered material is washed with water in order to remove excess salt and thereafter appropriately dried. This resulting powder is in essentially pure form and thereafter may be processed to prepare parts or bodies.

As previously noted, small amounts of certain materials may be added to the intermetallic in order to improve certain properties desired for specific uses. It has been found, for example, that the presence of aluminum in the final intermetallic composition improves the sinterability, low-temperature ductility and oxidation resistance. In addition, small amounts of materials such as titanium, boron and silicon improve oxidation resistance as well as high temperature stability. Accordingly, such materials may be added during the melting process in appropriate proportions, preferably up to about by weight of the total intermetallic composition or may be added to the powder prior to sintering or consolidation steps.

The consolidation or sintering steps required to process the powder into suitable parts are accomplished using standard techniques. The powder is placed in a vessel and thereafter into a sintering furnace. The furnace atmosphere is evacuated by vacuum, and heat and pressure are slowly applied. Typical conditions for the sintering operation include pressures between about 3,000 and 10,000 p.s.i. with temperatures being between about 2000 and about 3000 F. Thereafter, the material is cooled slowly and the final densified product achieved. A preferred vessel in which sintering takes place consists of a graphite die which has been lined with tantalum foil washed with a thin beryllium oxide coating. The tantalum prevents carbon contamination of the beryllide intermetallic while the oxide avoids reaction between the foil and the intermetallic.

The beryllides prepared by the process of the invention are essentially homogeneous compounds consisting of intermetallic compositions which are essentially pure and contain, within individual molecular structures, beryllides having the formulas set forth hereinabove. The compounds prepared by melting relatively large particles have superior properties over those prepared by other known methods. Thus, the intermetallics, rather than including a number of intermediate intermetallic beryllides other than those desired, comprise essentially the individual intermetallics of the formulas MBe and MBe and mixtures thereof.

The following example is illustrative of the manner in which the invention is carried out. The example is given by way of illustration only and specific conditions and compositions set forth therein are not to be considered as limiting the scope of the invention.

Example Ingots of columbium and beryllium each having an average diameter of about 1.5 inches were carefully weighed and placed in a beryllium oxide crucible. The amount of columbium was 198 grams and the amount of beryllium was 202 grams. The crucible was placed in an induction furnace which had been carefully purged with argon to remove essentially all oxygen and nitrogen present. The frequency utilized for the induction coil to melt the metals was about 10,000 cycles and the temperature was increased to about 3300 F. A slight argon pressure of 8 p.s.i. absolute was maintained during the melting operation. Upon complete melting of the two metals, the composition was stirred for about 1 to 5 minutes. Thereafter, the intermetallic was allowed to cool and removed from the crucible. The intermetallic beryllide ingot was analyzed by X-ray defraction methods in order to determine its composition. The composition consisted essentially of the compounds CbBe and Cb Be as expected from known phase relationships. The ingot was subjected to crushing and grinding and then placed in a ball mill in which steel balls were used to reduce the size of the intermetallic particles. The particles were thereafter screened to about 400 mesh and about 350 grams of the powder were placed in 1000 ml. of an aqueous solution of disodium ethylenediaminetetraacetic acid having a pH of about 5.0. The mixture was stirred for approximately 24 hours and filtered to recover the intermetallic particles. The powder was then washed with water and acetone and thereafter dried under vacuum. The powder was placed in a graphite die which was lined with a tantalum sheet coated with beryllium oxide film. The assembly was placed in a hot-press furnace within an induction coil and the furnace was pumped to a vacuum of about 1 micron Hg. The powder was then heated to between 2600 and 2900 F. under a pressure of about 4,000 p.s.i. for about 1 hour. Thereafter, the furnace was allowed to cool under vacuum and the final product removed. The beryllide was again subjected to X-ray defraction and found to consist of essentially the same composition as the ingot set forth above.

The beryllides prepared by the present process are found useful in preparing gas turbine nozzle vanes or rotating blades which have extremely resistant properties under high stress and corrosive environments. Other advantages and uses of the composition will be evident to those skilled in the art.-

We claim:

1. In a process for preparing intermetallic beryllides by heating a mixture consisting essentially of a refractory metal, M, and beryllium, Be, the improvement comprising melting particles of the metals having a particle size greater than 50 mesh in an inert atmosphere containing essentially no oxygen or nitrogen at a pressure between about 5 p.s.i. and about 10 psi. absolute and at a temperature of at least 3000 F.

2. The process of claim 1 wherein the average crosssection of the metal particles is at least about 0.1 inch.

3. The process of claim 1 wherein the equivalent weights of the respective metal particles M and Be is between about 1:8.5 and about 1:12 wherein the refractory metal is selected from the group consisting of Cb, Ta, Ti, V, Zr, Hi and Cr and wherein the resulting composition obtained from the melted particles comprises homogeneous intermetallic consisting essentially of MB MBe of mixtures thereof.

4. In the process of claim 1 wherein the resulting melted composition is cooled and processed into finely divided particles 200 mesh or smaller which particles are contaminated with iron, the improvement comprising treating the particles with an aqueous solution of References Cited UNITED STATES PATENTS 3,150,968 9/1964 Stonehouse et a1. 75150 3,150,970 9/1964 Beaver et a1. 75174 3,172,196 3/1965 Beaver et al. 29-182 3,172,761 3/1965 Beaver et al. 75150 L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner U.S. Cl. X.|R.

@ 5 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 $95,641 Dated July 27, 1971 Inventor(s) Milton S Roush et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 13, "MB should read --MBe line 14, "of" should read --or--,

Signed and sealed this 8th day of February 1972.

(SEAL) Attest:

EDWARD M.FLETCIER, JR BE T GOTTSCHALK Attesting Officer Commissioner of Patents 

